1. Field of the Invention
The present invention relates generally to a power control apparatus and method for a high-speed dedicated physical control channel in a mobile communication system supporting a high-speed downlink packet access (HSDPA) service, and in particular, to an apparatus and method for transmitting and receiving an uplink power offset value to transmit a high-speed dedicated physical control channel.
2. Description of the Related Art
HSDPA brings high-speed data delivery to user equipments (UEs) over a high-speed-downlink shared channel (HS-DSCH) and its related control channels. To support HSDPA, adaptive modulation and coding (AMC), hybrid automatic retransmission request (HARQ), and fast cell selection (FCS) have been proposed.
A. AMC
AMC is a technique for adapting a modulation and encoding format based on a received signal quality of a UE and a channel condition between a particular Node B and the UE to increase a use efficiency of an entire cell. Therefore, the AMC involves a plurality of modulation and encoding schemes (MCSs). MCS levels are set from level 1 to level n for AMC. In other words, the AMC is an adaptive selection of an MCS level according to the channel condition between the UE and the serving Node B.
B. HARQ
In HARQ, particularly n-channel SAW HARQ (n-channel Stop And Wait HARQ), two techniques are introduced to increase typical ARQ efficiency. That is, a retransmission request and a response for the retransmission request are exchanged between the UE and the Node B, and defective data is temporarily stored and combined with corresponding retransmitted data. The n-channel SAW HARQ has been introduced to overcome the shortcomings of conventional SAW ARQ in HSDPA. In the SAW ARQ, a next packet data is not transmitted until an acknowledgement (ACK) signal is received for previously transmitted packet data. This implies that even though the packet data can be transmitted, the ACK signal must be awaited. On the other hand, the n-channel SAW HARQ enables successive transmission of next packet data without receiving an ACK signal for transmitted packet data, thereby increasing channel use efficiency. If n logical channels are established between a UE and a Node B, and are identified by specific time or their channel numbers, the UE can determine a channel on which packet data has been transmitted at an arbitrary point in time. Also, the UE can rearrange packet data in a correct reception order or soft-combine corresponding packet data.
C. FCS
FCS is a technique for fast selecting a cell (hereinafter, referred to as a best cell) at the best condition among a plurality of cells when a UE supporting HSDPA is at a soft-handover zone, which is defined as an overlapped zone between Node Bs. When the UE enters the soft-handover region, it establishes radio links with the Node Bs. The cells of the Node Bs that have established radio links with the UE are the active set of the UE. The UE receives data only from the best cell in the active set, thereby reducing overall interference. The UE periodically monitors the channel conditions with the cells in the active set to determine if there is a cell better than the present best cell. If there is better a cell, the UE transmits a Best Cell Indicator (BCI) to the cells of the active set to change the best cell. The BCI contains an identifier (ID) of the new best cell. Upon receipt of the BCI, the cells determine whether the BCI indicates one of them. Then, the new best cell transmits an HSDPA packet to the UE on the HS-DSCH.
FIG. 1 schematically illustrates a conventional downlink channel structure of a mobile communication system supporting an HSDPA service (hereinafter, referred to as an “HSDPA mobile communication system”). Referring to FIG. 1, a downlink channel for a mobile communication system supporting HSDPA (hereinafter, referred to as a “HSDPA mobile communication system” for short) includes a downlink dedicated physical channel (DL_DPCH), a downlink shared control channel (DL_SHCCH) and an HS-DSCH.
The DL_DPCH transmits information required for an existing code division multiple access (CDMA) system (for example, a Rlease-99 system), and an HS-DSCH indicator (HI) indicating whether there is HSDPA packet data to transmit. The HI can be used in indicating SHCCH that a corresponding UE must receive the HSPDA packet data.
For example, when the HSDPA packet data is transmitted by the N (=N1+N2) slots (i.e., by the HSDPA transmission time interval (TTI)), the HI is partially transmitted for N1 slots and a part for transmitting the HI for the remaining N2 slots is subject to discontinuous transmission (DTX). However, if there is no HSDPA packet data to transmit, a part for transmitting HI of all slots constituting one TTI is DTX-processed. In this case, however, it is assumed that a slot format is fixed in the TTI. When the HSDPA packet data is transmitted by the 3 slots (i.e., one HSDPA TTI=3 slots), the HI is transmitted over a particular one of the 3 slots.
The SHCCH transmits control information needed by a corresponding UE to receive HSDPA packet data over the HS-DSCH. HS-DSCH control information transmitted over the SHCCH includes:
(1) Transport Format and Resource related Information (TFRI): this represents an MCS level and HS-DSCH channelization code information to be used for HS-DSCH, a size of a transport block set, and an identifier of a transport channel.
(2) HARQ information: this represents information needed to support HARQ.
(a) HARQ processor number: in n-channel SAW HARQ, this indicates a channel to which specific packet data belongs among logical channels for HARQ.
(b) HARQ packet number: in FCS, if a best cell is changed, this informs a UE of a unique number of downlink packet data so that the UE can inform a selected new best cell of a transmission state of the HSDPA data.
The SHCCH can be assigned one or two or more channelization codes. FIG. 1 illustrates an example where a maximum of 4 SHCCHs can be assigned. In this case, information on SHCCH that a corresponding UE must receive can be represented by a 2-bit HI. For example, the UE receives SHCCH#1 for HI=00, SHCCH#2 for HI=01, SHCCH#3 for HI=10; and SHCCH#4 for HI=11.
The HS-DSCH is used to transmit the HSDPA packet data. The HS-DSCH, because it transmits high-speed packet data, is assigned an orthogonal variable spreading factor (OVSF) code with a very low spreading factor (SF). For example, an SF=16 OVSF code can be assigned to the HS-DSCH.
A description will be made herein below of a process of receiving by the UE an HSDPA service using the above-stated three downlink channels of DL_DPCH, SHCCH, and HS-DSCH.
The UE receives a DL_DPCH signal and analyzes an HI field of the received DL_DPCH signal. If the HI field was DTX-processed, the UE waits until a next TTI without receiving any SHCCH, determining that there is no HSDPA packet data. However, if it is analyzed that the HI field has a specific bit value, the UE receives an SHCCH signal according to the specific bit value, determining that there is HSDPA packet data. Thereafter, the UE extracts an MCS level, channelization code information, and HARQ related control information necessary for demodulation of HS-DSCH, by decoding the received SHCCH signal. The UE receives an HS-DSCH signal, and demodulates and decodes the received HS-DSCH signal using the extracted control information.
As described above, in order to demodulate an HS-DSCH signal, the UE first determines control information by receiving a DL_DPCH signal and an SHCCH signal. Therefore, in FIG. 1, start points of the DL_DPCH and the SHCCH go in advance of a start point of the HS-DSCH.
FIG. 2 illustrates a structure of a DL_DPCH determined by adding an HI field for an HSDPA service to fields for an existing downlink data service. Referring to FIG. 2, files for the existing downlink data service include a first data field Data1, a transmit power control (TPC) field, a transmit format combination indicator (TFCI) field, an HI field, a second data field Data2, and a Pilot field. The first and second data fields Data1 and Data2 are used to transmit data supporting an operation of an upper layer or data supporting a dedicated service, such as a voice service. The TPC field transmits a downlink power control command for controlling transmission power of a UE, and the TFCI field transmits transmit format combination indicator information for the first and second data fields. The Pilot field transmits a pilot signal, as a previously agreed symbol stream, for estimating a condition of a downlink channel by a UE. The HI field for the HSDPA service can be created by puncturing, for example, a part of the first or second data field.
FIG. 3 schematically illustrates a structure of conventional uplink dedicated physical channels in an HSDPA mobile communication system. Referring to FIG. 3, an uplink dedicated physical channel includes an uplink dedicated physical data channel (UL_DPDCH), an uplink dedicated physical control channel (UL_DPCCH), and an uplink high-speed dedicated physical control channel (HS-DPCCH) for supporting an HSDPA service. The uplink dedicated physical channels are assigned unique channelization codes, for separate management. That is, for the HSDPA service, HS-DPCCH is defined as a new uplink control channel by assigning a channelization code instead of modifying the existing uplink control channel. This solves a compatibility problem with an existing system and a complexity problem of a channel structure, which may occur when the existing uplink channel is modified. The reason that HS-DPCCH can be newly assigned for the HSDPA service as stated above is because in the case of an uplink, channelization code resources are so sufficient that OVSF codes can be assigned to all UEs.
The UL_DPDCH transmits upper layer data by the slot (or slot by slot), and the UL_DPCCH transmits a Pilot symbol, TFCI bits, a feedback information (FBI) symbol, and a TPC symbol by the slot. The Pilot symbol is used as a signal for estimating a condition of an uplink channel by a Node B, and the TFCI bits represent a transmit format combination of data transmitted for a current frame. The FBI symbol represents feedback information when a transmit diversity technology is used, and the TPC symbol is used to control transmission power of a downlink channel. A spreading factor (SF) of OVSF codes used for the UL_DPCCH is fixed to 256.
The HS-DPCCH transmits a response signal based on whether an error has occurred in received high-speed packet data, and channel quality indicator (CQI) information. The response signal is divided into an acknowledgement signal (ACK) indicating non-occurrence of an error and a negative acknowledgement signal (NACK) indicating occurrence of an error. The CQI information is provided to a Node B by a UE to support AMCS for the HSDPA service. If it is assumed that 3 slots constitute one TTI, the ACK/NACK is transmitted over one of the three slots, and the CQI information is transmitted over the remaining two slots. Transmission of the ACK/NACK or CQI information is not required. The ACK/NACK field or CQI field can be subject to DTX during transmission.
FIG. 4 is a diagram illustrating a UE located in a handover state in a general 3rd generation asynchronous mobile communication system. It is assumed in FIG. 4 that 3 Node Bs exist in an active set of a UE, and of the 3 Node Bs, a Node B#1 405 and a Node B#2 406 belong to the same radio network controller (RNC) 402 and a Node B#3 420 belongs to another RNC 404. In FIG. 4, a radio network system (RNS) refers to an RNC and Node Bs controlled by the RNC in the 3rd generation asynchronous mobile communication standard. An RNS A 401 includes the RNC A 402 and its associated Node B#1 405 and Node B#2 406 controlled by the RNC A 402. RNS B 403 includes the RNC B 404 and its associated Node B#3 420 controlled by the RNC B 404. It is assumed herein that the RNC A 402 is a serving RNC (SRNC) and the RNC B 404 is a drift RNC (DRNC) 404. The “SRNC” refers to an RNC that manages a service of a corresponding UE and take charge of a connection with a core network (CN). Of RNCs handling data from the corresponding UE, all RNCs except an SRNC are called a “DRNC.”
A detailed operation performed by a UE in a handover state will be made with reference to FIG. 4. Referring to FIG. 4, a UE 419 moves away from a cell #1 407 while receiving an HSDPA service through downlink channels 411 of DL_DPCH SHCCH, and HS-DSCH. Of course, the UE 419 transmits DPDCH, DPCCH, and HS-DPCCH over an uplink. In this case, the UE 419 performs soft handover, if strength of a signal received from another cell along with a signal from the cell #1 407 is high enough. The UE 419 continuously monitors signals received from several cells and includes (or registers) cells with high signal strength in an active set. As a result, the UE 419 includes a cell #2 408, a cell #3 409 of the Node B#2 406 and a cell #4 of the Node B#3 420 in the active set, as illustrated in FIG. 4. As such, the UE 419 simultaneously receives signals from the other cells 408, 409, and 410 over DL_DPCHs 412, 413, and 414 along with the signal from the cell #1 407.
In this handover state, the UE 419 receives DL_DPCHs from the other cells #2, #3, and #4 in the active set as well as DL_DPCH, SHCCH, and HS-DSCH from the cell #1 407. That is, the UE 419 receives SHCCH and HS-DSCH for the HSDPA service only from the cell #1 407. This is because the HS-DSCH does not support soft handover. The reason is because in implementation, it is difficult for the other Node Bs 406 and 420 to analyze a packet data transmission state of the Node B#1 that transmits high-speed data, and subsequently transmit a data packet. The UE 419 performs soft combining on DL_DPCHs from the four cells 407, 408, 409 and 410, for analysis. The term “soft combining” refers to receiving by the UE 419 signals on different paths through corresponding fingers and combining the received signals. The soft combining attempts to reduce an influence of noises affecting a received signal by summing up the same information received through different paths, analyzing the summed information, and providing a multipath diversity effect for the received signal to the UE. In a mobile communication system, power control is generally performed on channels between a Node B and a UE. However, power control over HS-DPCCH proposed for supporting the HSDPA service is not separately performed, but performed in the same manner as power control over the UL_DPCCH. In other words, DPCCH and HS-DPCCH have a constant power ratio, and if transmission power of the UL_DPCCH is increased or decreased due to power control, transmission power of the HS-DPCCH is also increased or decreased. The transmission power of the UL_DPCCH is controlled by TPC, a power control command transmitted over a TPC field of DL_DPCH. A problem on power control over HS-DPCCH, which may occur because of performing power control depending on a ratio of transmission power of the HS-DPCCH to transmission power of the DPCCH, will now be described with reference to FIG. 4.
Describing a common uplink power control process in the existing Release-99, a Node B receives a Pilot signal over UL_DPCCH and measures an uplink signal-to-interference ratio (SIR) by the received Pilot signal. The Node B compares the measured SIR with a target SIR and transmits TPC over DL_DPCH according to the comparison result. For example, if the measured SIR is lower than the target SIR, the Node B transmits to a UE a command for increasing uplink transmission power (hereinafter, referred to as a “power-up command”) over a TPC field of DL_DPCH. However, if the measured SIR is higher than the target SIR, the Node B transmits a command for decreasing transmission power (hereinafter, referred to as a “power-down command”).
Describing power control over an uplink channel in a handover state, a UE receives TPCs over DL_DPCHs from all Node Bs included in an active set. If at least one of the received TPCs includes a power-down command, the UE decreases transmission power of an uplink channel. For example, if the UE receives a power-up command from the cell #1 407 and power-down commands from the other Node Bs 406 and 420, the UE 419 decreases transmission power of the uplink channel. That is, even though the cell #1 407 supporting an HSDPA service continuously transmits a power-up command, the UE will decrease transmission power of the UL_DPCCH. This means that transmission power of the HS-DPCCH in power control is also decreased while maintaining a constant power ratio to the UL_DPCCH. The reason that such power control has not raised any problem conventionally is because the UL_DPDCH and the UL_DPCCH are transmitted to all cells within a handover region, so the RNC A 402, an upper layer, can perform combining. However, as the HS-DPCCH for the HSDPA service is received through only one cell 407, the RNC A 402 cannot perform combining. Therefore, the above-stated uplink power control may undesirably decrease reliability of the HS-DPCCH that transmits ACK/NACK and CQI information, which are important to the HSDPA service.
In order to solve this problem, it is necessary to provide a power control method different from the existing uplink power control when a UE is located in a handover region. For example, the UE transmits HS-DPCCH at transmission power increased by a predetermined value against the transmission power of UL_DPCCH.
To this end, a Node B compares a measured SIR with a target SIR, and determines that a UE is located in a handover region or has a poor channel condition, if the measured SIR is lower by a threshold value or higher than the target SIR. Thereafter, the Node B defines a difference between the measured SIR and the target SIR as an uplink power offset value of HS-DPCCH, and transmits the uplink power offset value to the UE. The UE then increases transmission power of HS-DPCCH by the uplink power offset value before transmission.
In order to perform power control as stated above, a detailed definition should be made of a method for determining the uplink power offset value and a method for transmitting the determined uplink power offset value to a UE. Generally, there is a method for transmitting by a Node B an uplink power offset value to a UE over a particular field of a physical channel. However, this method should always assign a fixed field to a physical channel even when it is not necessary to transmit an uplink power offset value, causing a decrease in utilization efficiency of resources.